The limits of sexual selection

What limits the size of a peacock's tail, the weight of a deer's antlers or the virtuosity of a songbird's song? Driven inexorably by the competition to attract mates, these features of animals ought to get ever more elaborate. There was even once a theory-now discredited-that the famously gigantic antlers of the Irish elk became so unwieldy that they caused its extinction. Yet sexual ornaments do not get ever bigger.

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John S. Dykes

Now comes evidence from studies of peacocks and frogs that what sets the limit is the ability of females to perceive any difference between one level of elaboration and the next. Cutting the tails of peacocks short prevents them from attracting peahens, but having a slightly larger tail than a rival does not make much difference, because the peahen cannot tell the difference. Likewise, female tungara frogs are attracted to males with complex songs but cannot tell the difference between elaborate songs and slightly more elaborate ones.

Let me back up. The philosopher Herbert Spencer turned Charles Darwin's theory of natural selection into a sound bite: "survival of the fittest." But in his later book, "The Descent of Man," Darwin had a second theory: sexual selection. The sound-bite version of sexual selection is "reproduction of the sexiest."

In other words, some features of animals don't help the individual to survive-and may even hinder its survival-but they do help it to reproduce, either by enabling it to compete against rivals or by attracting members of the opposite sex. Darwin's idea of sexual selection by female choice was all but abandoned by biologists in the ensuing century. It seems that the idea of active female sexual discrimination made male biologists uncomfortable.

"Darwin posited the idea of sexual selection, the sound-bite version of which is 'reproduction of the sexiest.'"

By the 1970s, however, a series of experiments had conclusively shown that Darwin was right and the ornaments, colors and songs of (mostly) males had been selected by the persistent and idiosyncratic preferences of (mostly) females. Once such a habit of sexual selection gets going, a female that bucks the trend risks having sons that cannot attract other mates.

In short, sexual selection seems to be a runaway process, like a nuclear chain reaction: The more it occurs, the more it will occur. Certainly it can produce bizarre results, burdening males with huge plumes, exhausting dances or arduous songs. But it does not seem to accelerate like a chain reaction. Peacocks' tails are not getting bigger and bigger in each generation. They got big and then stopped.

The explanation seems to lie in the neuroscience of perception. As Mike Ryan of the University of Texas at Austin puts it, just as you can easily tell the difference between a pile of six oranges and a pile of five, but you cannot easily distinguish 100 oranges from 101 oranges, so a female tungara frog in Panama can tell the difference between a simple male song and an elaborate one, but not between an elaborate and a very elaborate one. The song has evolved to the limit of female discrimination.

An alternative explanation for the lack of runaway sexual selection is that predators knock off the most elaborately burdened males, counterbalancing the females' pressure. But Dr. Ryan's experiments with tungara frogs prove that this is not the case. The frog's main predator is a bat, which homes in on the frog's song, and the bat is like the female frog: It shows no preference for males whose songs are not just elaborate but very elaborate.

A recent paper by Roz Dakin and her colleagues at Queen's University in Ontario comes to a similar conclusion about the plumage of peacocks. The difference between the best and the good peacock tail is too subtle for peahens to perceive. This news should encourage the great multitude of peacocks whose plumage is merely average.